In the late 21st century, practical nanotech became a reality. Then it mutated and got loose. Or it was stolen and used as a weapon. No one is really sure, since by the time it was brought under control virtually all of Earth's infrastructure had been destroyed.
That was five generations ago. Humanity still survives, the crippled biosphere supported by protective nanotech that shields it from the pervasive presence of the "eaters". With advanced tech impossible to keep protected and operational, people have turned inward, Shaping themselves with nanotech to become the tools they can no longer manufacture.
Meanwhile, the eaters have also evolved, clustering together into vast consuming intelligences. Can the Age of Ruin be turned into the Age of Redemption, or will the fifth generation after the Fall of Man be his last?